aperture coupling
Hi, antenna_abc:
A typical aperture coupled antenna is like the following:
-----------------patch
air or foam
------------------
substrate
-----aperture on ground----------------
substrate
--------------------feed line
Normally, people want to have a thick layer of air of foam. From what I see, the big space there allows the antenna to store energy. Then, the antenna has larger bandwidth. I am not sure about "Chu-Criterion for small antenna" and I could not comment on it.
From what I see, bigger space to store energy should improve the bandwidth. A dipole antenna has wider bandwidth than a typical patch because it has more space. An oval shaped antenna even has wider bandwidth because it occupies even more space.
Regarding tuning network, it can make a structure wider bandwidth or narrower bandwidth. It really depends upon how you make it. In fact, for any passive structure with no matter how little radiation, as long as Re(Zin) > 0, you can find a response of a passive network to make its bandwidth to be infinity. However, you may not be able to synthesize the passive network because it normaly is an anti-phase element over a very wide frequency range or the ANG(S21) is increasing with frequency. A normal passive network's ANG(S21) is decreasing with frequency except near resonances. The difficulty to synthesize a network with increasing ANG(S21) in a wide band is not smaller than tuning an antenna element for wider bandwidth. Regards.